The present invention relates to the art of image processing. It finds particular application in conjunction with mapping full color images to textured highlight color images, and, more particularly, to retaining luminance, saturation, and hue information from full color images in the reduced color gamut of a highlight color scheme with the use of textured patterns. While described with reference to particular image reading and rendering engines (e.g., digital copiers and/or printers, video monitors, highlight color printers, laser and/or ink jet color printers, data reading and/or storage devices, scanners, etc.) it is to be appreciated that the present invention is also amenable to other like applications.
Color images have become a significant element in the printing industry and are used regularly in video presentations, hard copy reports, and otherwise. As a result, color image processing tools including reading and rendering engines with color processing capabilities are in demand and have been developed to obtain color images. Much of the color rendering is performed using a full color gamut which includes tints and shades of the full color spectrum, e.g. reds, greens, blues and their combinations. The gamut of colors in a full color image can be described as a three-dimensional (3D) volume which is represented by the double hexagonal cone 10 illustrated in FIG. 1. In the cone 10, luminance varies from dark to light as one moves upward along a vertical axis 12, saturation varies from unsaturated grays to fully saturated colors as one moves radially outward from the vertical axis 12, and hue varies as one moves angularly about the vertical axis 12.
Nevertheless, a significant amount of color rendering can be adequately performed using a highlight color scheme. The use of highlight color rendering over full color rendering is attractive for a number of reason. For example, it is advantageous to use a highlight color printer because it is generally faster and less expensive than a full color printer since only two inks are put down as opposed to the three or four inks which must be used in order to obtain full color images. However, in the highlight color scheme, a restricted gamut of colors is available to the rendering engine. As pointed out for example, in highlight color printing, only two inks are used in the printing process. These inks typically comprise a base color (i.e., black or white) and a highlight color (e.g., red, blue, green, or other non-grayscale color). The restricted gamut can be represented by the two-dimensional (2D) triangle 14 illustrated in FIG. 2. The triangle 14 is a vertical slice from the full color cone 10 in FIG. 1 taken along the axis 12 and extending radially outward therefrom at the angle corresponding to the hue of the highlight color.
Color image processing and/or rendering that involves mapping from a 3D color gamut to a restricted gamut has been addressed in commonly owned U.S. Pat. Nos. 4,903,048 and 5,153,576 to Harrington, and commonly owned U.S. Pat. No. 5,237,517 to Harrington, et al., all incorporated herein by reference. U.S. Pat. No. 5,153,576 is concerned with mapping color images to black-and-white using textured patterns in such a manner that the various colors in a 3D color space are mapped to distinct textures. U.S. Pat. Nos. 4,903,048 and 5,237,517 are concerned with color image processing including mapping from a full color gamut to a highlight color gamut, and rendering of images in a highlight color gamut.
Referring again to FIGS. 1 and 2, rendering a full color image on a highlight color rendering engine involves mapping coordinates from the 3D double hexagonal cone 10 into the 2D triangle 14. Accordingly, the rendering engine is called upon to reproduce as closely as possible the full color image with the reduced color gamut available to it while minimizing the loss of information (e.g., luminance, saturation, and/or hue) contained in the original full color image. In previously developed mapping techniques, typically many different colors (defined by their coordinates in the full color space) are mapped to the same location in the highlight color space. This results in the loss of important hue information when mapping the full color image to the highlight color scheme. That is to say, with some prior developed mapping techniques, different colors (e.g., yellow and magenta) which have the same luminance and same saturation level of highlight color (e.g., red) are indistinguishable when mapped to the highlight color gamut.
The loss of hue information is particularly disadvantageous in the case of functional or presentation graphics. Color images associated with presentation graphics, use color differently from pictorial images. In presentation graphics, the colors chosen are often bright and highly saturated for maximum visual impact. Specific items or images in presentation graphics (e.g., bars on a graph, slices of a pie chart, etc.) tend to be differentiated primarily by hue. Consequently, important information lies in the hue.
Accordingly, the present invention contemplates a new and improved mapping technique which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of color image processing for mapping an input image from a full color gamut to a textured highlight color gamut is provided. The method includes defining an input color in a desired three dimensional color space by determining intensity values for a first color coordinate, a second color coordinate, and a third color coordinate. A halftone cell is partitioned into first and second regions. The first region contains first pixels, and the second region contains second pixels. Based on the intensity values, a first number of first pixels are selected for rendering in a highlight color. The first number of first pixels are selected from all the first pixels in a first sequential order. Again based on the intensity values, a second number of first pixels are selected for rendering in a base color. This time the second number of first pixels are selected from all the first pixels in a second sequential order. Based on the intensity values, a first number of second pixels are selected for rendering in the highlight color with the first number of second pixels being selected from all the second pixels in a third sequential order. Also based on the intensity values, a second number of second pixels are selected for rendering in the base color, and the second number of second pixels are selected from all the second pixels in a fourth sequential order.
In accordance with another aspect of the present invention, a color image processing system includes an image input terminal which reads an input image represented in a fill color gamut, and an image output terminal which renders the input image in a textured highlight color gamut. Also included is an image processing unit which samples colors from the input image and maps the sampled colors to textured highlight color patterns.
One advantage of the present invention is that it retains hue information from an original full color image when mapping the full color image to a highlight color image by using textured patterns in the highlight color image to represent different hues from the full color image.
Another advantage of the present invention is that the relative luminance and saturation properties are retained in mapping from full color images to textured highlight color images.
One other advantage of the present invention is that its a high quality, fast, and cost efficient technique for rendering full color images with a rendering engine having a restricted gamut of colors available thereto.
Yet another advantage of the present invention is that it provides for an arbitrary selection of the hue used for the highlight color.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.